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Re: [PATCH v2] util/async: Add memory barrier to aio_ctx_prepare

From: Paolo Bonzini
Subject: Re: [PATCH v2] util/async: Add memory barrier to aio_ctx_prepare
Date: Thu, 2 Apr 2020 10:47:08 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 02/04/20 04:44, Ying Fang wrote:
> Normal VM runtime is not affected by this hang since there is always some
> timer timeout or subsequent io worker come and notify the main thead.
> To fix this problem, a memory barrier is added to aio_ctx_prepare and
> it is proved to have the hang fixed in our test.

Hi Ying Fang,

this part of the patch is correct, but I am not sure if a memory barrier
is needed in aio_poll too.

In addition, the memory barrier is quite slow on x86 and not needed there.

I am sorry for dropping the ball on this bug; I had a patch using
relaxed atomics (atomic_set/atomic_read) but I never submitted it
because I had placed it in a larger series.  Let me post it now.



> diff --git a/util/async.c b/util/async.c
> index b94518b..89a4f3e 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -250,7 +250,8 @@ aio_ctx_prepare(GSource *source, gint    *timeout)
>      AioContext *ctx = (AioContext *) source;
>      atomic_or(&ctx->notify_me, 1);
> -
> +    /* Make sure notify_me is set before aio_compute_timeout */
> +    smp_mb();
>      /* We assume there is no timeout already supplied */
>      *timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));

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